The ability of natural killer (NK) cells to kill abnormal cells and produce cytokines and chemokines allows them to participate in the initial defense against viruses and tumors, as well as influence the subsequent adaptive immune response. In order to prevent inadvertent tissue destruction, NK cells not only undergo an "education" process to ensure they are tolerant to healthy "self" cells, but also display adaptive properties that respond to environmental influences and modulate NK cell effector response. We demonstrated that continuous engagement of an activating receptor on the NK cell results in an induced hyporesponsiveness (tolerance). Those studies utilized a novel transgenic mouse (m157-Tg) that expresses the murine cytomegalovirus (MCMV) encoded m157 protein (the only known ligand for the NK cell-specific activating receptor Ly49H) and demonstrated a hyporesponsiveness in the Ly49H+ NK cell population. Tolerant Ly49H+ NK cells in the m157-Tg mouse were characterized by impaired cytokine production and degranulation in response to stimulation through non-Ly49H activating receptors. The mechanism(s) by which activating receptors modulate NK cell adaptive responses (induction of tolerance versus activation) are not understood. In this proposal we will investigate the role of activating receptor engagement on NK cell adaptation. The core hypothesis to be examined in this proposal is that persistent stimulation through NK cell activating receptors alters signal transduction through the activating receptor signaling pathway, resulting in a tolerant rather than activated state of the NK cell. The specific aims to this hypothesis will (1) determine the mechanism by which Ly49H engagement delivers an inhibitory signal to NK cells, (2) evaluate signaling changes in non-Ly49H activating receptors from tolerant NK cells, and (3) define the context by which m157/Ly49H interaction results in NK cell tolerance in vivo. PUBLIC HEALTH RELEVANCE: Natural killer (NK) cells circulate through the blood and are present in virtually all organs, where they are poised to eliminate stressed, virally infected, or transformed cells without prior sensitization. These attributes have led to attempts to design potential NK-cell-based immunotherapeutic strategies against human cancer and viral infections. Recent studies suggest that NK cells have the capacity to adapt to their environment and alter their effector response. Studies described in this proposal will help us understand mechanism(s) of NK cell adaptation, which will be essential if we are to harness the capabilities of NK cells in future immunotherapeutic strategies.